Give EDA_SHAPE a virtual destructor

EDA_SHAPE can be inherited from, so a virtual destructor is needed
to ensure the destructors are called in order.

include/eda_shape.h

@@ -69,7 +69,7 @@ public:
     // Do not create a copy constructor & operator=.
     // The ones generated by the compiler are adequate.
 
-    ~EDA_SHAPE();
+    virtual ~EDA_SHAPE();
 
     void SwapShape( EDA_SHAPE* aImage );